Process and apparatus for plasma sterilizing with pulsed antimicrobial agent treatment

ABSTRACT

A process and equipment for plasma sterilization in which articles in a sterilizing chamber are taken through at least one combination sterilizing cycle. The articles to be sterilized may include the interior of a vessel or chamber. In that case, the vessel or chamber itself may serve as a sterilizing chamber and connects to the plasma generating chamber. Each combination sterilizing cycle includes a pulsed treatment with gaseous antimicrobial agent, removal of the gaseous antimicrobial agent, and a plasma treatment. The pulsed treatment includes one or more pulse-vacuum cycles, each pulse-vacuum cycle includes the steps of evacuating the sterilizing chamber and exposing the article to the gaseous antimicrobial agent with a predetermined pressure profile during a predetermined time. The gaseous antimicrobial agent is preferably carried in a gas mixture with a nonreactive carrier gas. In one embodiment, the predetermined pressure is pulsed. In another embodiment, it is ramped. After the pulsed treatment, the antimicrobial agent is removed by evacuating the sterilizing chamber. The plasma treatment includes exposure of the article to a plasma having essentially uncharged, highly reactive free radicals, molecules and atoms.

This is a continuation of application Ser. No. 08/861,956, filed May 22,1997, now abn., which is a continuation of application Ser. No.08/461,412, filed Jun. 5, 1995, now U.S. Pat. No. 5,645,796, which is acontinuation-in-part of application Ser. No. 08/266,129, filed Jun. 27,1994, now abandoned, which is a division of application Ser. No.08/065,859, filed May 21, 1993, now U.S. Pat. No. 5,413,758, which is acontinuation-in-part of application Ser. No. 07/749,041, filed Aug. 27,1991, now U.S. Pat. No. 5,244,629, which is a continuation-in-part ofapplication Ser. No. 07/576,235, filed Aug. 31, 1990, now U.S. Pat. No.5,084,239.

FIELD OF THE INVENTION

This invention relates to a plasma sterilization process and apparatuscomprising pulsed treatment with a gaseous or vaporized antimicrobialagent such as hydrogen peroxide or a peracid to kill microorganisms andspores on the article. In particular, this invention relates to exposingan article to be sterilized to a plurality of treatment cycles, eachcycle including cyclic pulses of a gaseous or vaporized antimicrobialagent at one pressure, followed by pressure reduction to a lowerpressure. The article is then exposed to a gas plasma. The article to besterilized may include a container or enclosure whose interior isrequired to be sterilized.

BACKGROUND OF THE INVENTION

A variety of gas sterilization methods has been investigated in thepast. Methods using ethylene oxide and other disinfecting gases arewidely used for sterilizing a wide range of medical products frompharmaceutical preparations to surgical instruments. Irradiation aloneor together with disinfecting gases has also been investigated, assummarized by Russell, A. THE DESTRUCTION OF BACTERIAL SPORES. New York:Academic Press (1982).

A sterilizing method must effectively kill all organisms, includingspores, without damage to the article or goods being sterilized.However, many disinfecting gases which meet this criterion, such asethylene oxide and irradiation methods have been recognized to exposeworkers and the environment to safety hazards. States and Federallegislation are severely restricting the amount of hazardous gases suchas ethylene oxide (a carcinogen) in the working environment, or the useof any system or method which produces toxic residues or exhaustproducts. This is presenting a major crisis in hospitals and other areasof the health industry.

DESCRIPTION OF THE PRIOR ART

Sterilizing plasmas have been generated with a wide variety of gases:argon, helium or xenon (U.S. Pat. No. 3,851,436); argon, nitrogen,oxygen, helium or xenon (U.S. Pat. No. 3,948,601); glutaraldehyde (U.S.Pat. No. 4,207,286); oxygen (U.S. Pat. No. 4,321,232); oxygen, nitrogen,helium, argon or Freon with pulsed pressure (U.S. Pat. No. 4,348,357);hydrogen peroxide (U.S. Pat. Nos. 4,643,876 and 4,756,882); nitrousoxide, alone or mixed with oxygen, helium or argon (Japanese ApplicationDisclosure No. 103460-1983); and nitrous oxide, alone or mixed withozone (Japanese Application No. 162276-1983). Unfortunately, theseplasmas have proven to be too corrosive to articles being sterilized andparticular packaging materials; have left toxic residues on thesterilized articles; or have presented safety or environmental hazards.

Typical prior art plasma sterilizing systems such as U.S. Pat. No.4,643,876 have a combined chamber where both plasma generation andsterilization take place. The plasma is generated from hydrogen peroxidevapor and residue, and the article being sterilized is directly exposedto the plasma inducing electromagnetic field. The in situ generation ofthe ions and free radicals in the vicinity of the article surface isconsidered to be a critical part of the static process. Antimicrobialhydrogen peroxide pretreatment has been combined with exposure of thearticle to the electromagnetic plasma generating environment to removeany remaining hydrogen peroxide residues. The process is static, thatis, the plasma is generated in the volume of gas initially in the closedchamber, and the articles are not exposed to plasma generated from amixture of hydrogen, oxygen and inert gases, as in the process of thisinvention. These systems tend to rapidly decompose plastic and cellulosecontaining packages because of the strong oxidizing properties of theions and free radicals in the elevated temperatures of the process.Limiting the process time to prevent package destruction also producesan inadequate spore kill rate.

Plasma gas sterilizer systems described in U.S. Pat. Nos. 3,851,436 and3,948,601 comprise separate plasma RF generation chamber and sterilizingchamber. A gas plasma produced in the plasma generating chamber withargon, helium, nitrogen, oxygen or xenon is passed into a separatesterilization vacuum chamber.

Non-plasma gas sterilization procedures have been described using ozone(U.S. Pat. No. 3,704,096) and hydrogen peroxide (U.S. Pat. Nos.4,169,123, 4,169,124, 4,230,663, 4,366,125, 4,289,728, 4,437,567 and4,643,876). These materials are toxic or corrosive and leave undesirableresidues.

Peracid sterilization processes have been disclosed in East GermanPatent Application Serial No. 268,396, EPO Patent ApplicationPublication No. 109,352 A1, and U.K. Patent 2,214,081, for example. Thesporicidal activities of peracetic acid, alone and in combination withother compounds including ethanol and hydrogen peroxide are disclosed byLeaper, S., Food Microbiology. 1:199-203 (1984); Leaper, S. et al, J.Applied Biol. 64:183-186 (1988); Leaper, S., J. Food Technology.19:355-360 (1984); and Leaper, S., J. Food Technology. 19:695-702(1984). These methods are not effective to sterilize the contents ofpackages containing cellulose and other materials which are reactivewith peracid species.

The use of plasma to sterilize containers was suggested in U.S. Pat. No.3,383,163.

The above apparatus and methods do not achieve complete sterilizationfor many types of articles requiring sterilization, and most producedamage to articles and packaging in the course of producing highsterilization rates. As a result, they do not achieve the necessary goalof providing an all purpose, effective sterilizing system and process.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved plasmasterilizing process which carries out effective sterilization quickly,with no toxic residues and with emissions which present no environmentalsafety hazard and without damage to articles, including those that arepackaged or in the form of a container.

It is another object of this invention to provide an economicalsterilizing process which is safe and effective for use in a hospitalenvironment.

It is another object of the present invention to provide an efficientprocess which achieves sterilization with all types of articles used inthe health care environment, including metallic articles and articlescontained in porous sterilization packaging including cellulosicmaterials.

It is also another object of the present invention to provide anefficient and cost-effective process for sterilization of vessels orchambers such as lyophilizers or sterile isolation enclosures.

These and additional objects are accomplished by the method of thisinvention for plasma sterilization which comprises exposing an articlein a sterilizing chamber to at least one combination sterilizing cycle.In the case where the article to be sterilized is the interior of acontainer, the container itself may function as a sterilizing chamberconnectable to a plasma source and an antimicrobial source. Eachsterilizing cycle is comprised of a pulsed treatment with gaseousantimicrobial agent, removal of the gaseous antimicrobial agent, and aplasma treatment. The pulsed treatment comprises one or morepulse-vacuum cycles, each pulse-vacuum cycle comprising the steps ofevacuating the sterilizing chamber and exposing the article to thegaseous antimicrobial agent for a predetermined duration. After thepulsed treatment, the antimicrobial agent is removed by evacuating thesterilizing chamber. The plasma treatment comprises exposing the articleto a stream of plasma having essentially uncharged, highly reactive freeradicals which are oxidizing or reducing agents. The plasma is generatedin a separate plasma generating chamber and is supplied to effectsterilization in the sterilization chamber. The pulsed treatment and theplasma treatment follow a predetermined order in each combinationsterilizing cycle.

According to one aspect of the invention, the antimicrobial agent ispreferably selected from the group consisting of hydrogen peroxide, aperacid antimicrobial agent, or mixtures thereof, the peracidantimicrobial agent being selected from the group consisting ofsaturated and unsaturated peralkanoic acids having from 1 to 8 carbonatoms and halogenated derivatives thereof According to another aspect ofthe invention, the pressure of the gaseous antimicrobial agent is rampedup (i.e. monotonically increased) during the predetermined duration.

According to another aspect of the invention, the gaseous antimicrobialagent is introduced in a gaseous mixture with a nonreactive carrier gas.

According to another aspect of the invention, the gaseous mixture ismaintained at substantially a maximum concentration supported by thetemperature maintained in the sterilization chamber.

According to yet another aspect of the invention, the gaseousantimicrobial agent in the gaseous mixture has a partial pressuresubstantially at its saturation vapor pressure supported by thetemperature maintained in the sterilization chamber.

According to another aspect of the invention, water vapor is mixed withthe gaseous antimicrobial agent to enhance its sterilizing action.

A plasma source gas mixture is ionized into a plasma having ionizationproducts that include highly destructive components in the form ofcharged particles and ultra-violet radiation. Preferably, sterilizationis effected by employing plasma downstream products consistingessentially of uncharged, highly reactive free radicals, atoms andexcited molecules of a gas mixture to sterilize articles. A plasmadistribution device blocks the ultra-violet radiation and facilitatesthe recombination of the charged particles such that essentiallyuncharged, highly reactive free radicals, atoms and excited molecules ofthe gas mixture are delivered to the articles or vessels to besterilized.

This combination sterilizing cycle treatment process is particularlysuitable for sterilizing a porous article or an article enclosed in aporous container, the container being surrounded by the gas plasmaduring the treatment, even when the porous container comprises acarbohydrate composition.

According to yet another aspect of the present invention, the article tobe sterilized is the interior of a vessel or a chamber and any articlestherein. One example of such a chamber would be a sterile isolationenclosure or a lyophilizer which functions as a vacuum chamber forfreeze-drying pharmaceutical and medical products. In this application,the lyophilizer chamber is adapted to receive a gas plasma from a plasmagenerating chamber so that it acts as a sterilization chamber where itsinterior is to be sterilized. This method is effective and economicalsince conventional sterilization methods using ethylene oxide or steamhave their drawbacks, and the lyophilizer being built as a vacuumchamber expediently allows adaptation to a plasma sterilizer without theexpense of a separate vacuum system.

Additional objects, features and advantages of the present inventionwill be understood from the following description of the preferredembodiments, which description should be taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a plasma sterilizer suitable for use in theprocess of this invention.

FIG. 2 is a front view of the plasma sterilizer embodiment of FIG. 1.

FIG. 3 is a cross-sectional view of the plasma sterilizer embodiment ofFIG. 1 and FIG. 2, taken along the line 3—3 in FIG. 2.

FIG. 4 is a cross-sectional view of the plasma sterilizer embodiment ofFIG. 3, taken along the line 4—4.

FIG. 5 is a cross-sectional view of tube 54 taken along line 5—5 in FIG.3.

FIG. 6 is a cross-sectional view of tube 58 taken along line 6—6 in FIG.3.

FIG. 7 is a cross-sectional view of tube 56 taken along line 7—7 in FIG.3.

FIG. 8 is a schematic illustration of adapting a lyophilizer forsterilization.

FIG. 9 is a schematic illustration of a general enclosure adapted to besterilized by the plasma process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hospitals originally relied on disinfectants and steam autoclaves forsterilizing implements. In more recent years, ethylene oxide gassterilization has made possible the sterilization of heat labilepackaged articles, drugs and medical supplies, and hospital systems arehighly dependent upon these procedures. However, ethylene oxide is nowknown to be a dangerous carcinogen, and a number of new state lawsprotecting worker safety and the environment are precluding further useof ethylene oxide sterilizers in hospital environments.

Numerous gas plasma sterilizers using a wide variety of gases have beendescribed in the literature. A few have been commercially produced. Onesystem described in. U.S. Pat. No. 4,643,876, for example, pretreats thearticle to be sterilized with hydrogen peroxide before it is placed inthe electromagnetic field producing the plasma. It relies on thepresence of the hydrogen peroxide in the electromagnetic field as asource of the plasma products and the direct exposure of the hydrogenperoxide to the electromagnetic field to destroy the hydrogen peroxide.This system is suitable only for sterilizing non-metallic articlesbecause of the heating-of metallic articles and the destabilizing effectof metallic articles in plasma generating electromagnetic fields.However, even with hydrogen peroxide pretreatment, completesterilization is not achieved without severe degradation of thepackaging materials.

A few have focused on residue contamination problems. The previouslydescribed gas sterilizers either fail to satisfy current regulatoryresidue and exhaust emission safety standards of several states, becausethey either leave unacceptable residues, produce exhaust emissions whichare potentially hazardous to hospital personnel, or cause unacceptabledestruction of packaging materials. Substituting one hazard for another,they are thus not satisfactory for replacing ethylene oxide sterilizers.

Peracids such as peracetic acid are well known as sterilizing agents insituations where their residues can be tolerated or easily removed andwhere adequate exposure time is allowed. However, peracid vapors arealso known to be ineffective for sterilizing goods packaged in theconventional cellulosic sterile packages used in the health care field.

This invention is based on the discovery that pulsed antimicrobial agentpretreatment of both packaged and unpackaged articles, followed byexposure of the articles to sterilizing gas plasmas, reliably and moreefficiently kills resistant spores at conditions which are notdestructive to packaging materials. The exposure to the gas plasma in asterilizing chamber separate from the plasma generating system protectsthe packaging and plastic components and permits the sterilization ofmetallic articles.

The process of this invention employs a plasma produced from gasmixtures containing essentially oxidizing agents such as oxygen and/orreducing agents such as hydrogen, and optionally other carrier gasessuch as inert gases. The exhaust gas products fully satisfy currentenvironmental and worker safety concerns, the products of the plasmabeing almost entirely water vapor, carbon dioxide and non-toxic gasesnormally found in the atmosphere.

The term “plasma” as used herein is defined to include any portion ofthe gas or vapors which contain electrons, ions, free radicals,dissociated and/or excited atoms or molecules produced as a result ofthe applied electric or electromagnetic field including any accompanyingradiation which might be produced. The electromagnetic field can cover abroad frequency range, and can be produced by a magnetron, klystron orRF coil. For purposes of clarity of presentation and not by way oflimitation, the description hereinafter describes the use of a magnetronas the electromagnetic field source, and the use of all other suitablesources of the electromagnetic field required for plasma production areintended to be included, including without limitation, magnetrons,klystron tubes, RF coils, and the like.

The term “sterilization” connotes a process by which all viable forms ofmicroorganisms are destroyed or removed from an object. Sincemicroorganisms die according to first order chemical kinetics, it iscustomary to define sterility in terms of “probability of survivors”.The practical goal of a sterilization process is therefore measured as aprobability (e.g., 10⁻³, 10⁻⁶, 10⁻¹²), the probability indicating thelethal effect of a particular sterilizing dose or regimen. It is usualto assume increased time of exposure to a set of sterilizing conditionswill decrease the probability of survivors accordingly. Doubling thesterilizing time of identical conditions would result in a doubling ofthe exponent of the probability term, for example 10³¹ ⁶ would become10⁻¹².

The term “pretreatment” is used herein to define that at least onepulsed treatment of the article being sterilized with antimicrobialagent is followed by treatment with gaseous plasma products. The“pretreatment” with antimicrobial agent can follow one or more earlierplasma treatments and can be followed by one or more plasma treatments.Repetitions of the pulsed treatment and plasma gas treatment cycle anynumber of times can be used until total killing of spores with even themost resistant articles is achieved. The combination of peroxide and/orperacid antimicrobial agent and plasma gas treatments is synergistic inachieving a spore kill rate which exceeds the killing rate which can beachieved by use of hydrogen peroxide or peracid alone, or plasma gasesalone, while preserving the integrity of packaging materials. Theresidues are also entirely eliminated by the plasma gases and vacuum.

The pulsed treatment comprises one or more pulse-vacuum cycles. Eachpulse-vacuum cycle preferably begins by evacuating the sterilizationchamber. This is followed by exposing the article to be sterilized inthe sterilization chamber to gaseous antimicrobial agent at apredetermined pressure for a predetermined period. If the antimicrobialagent exists in a liquid state at room temperature, it is firstvaporized. For considerations of stability, ease of transport, andhigher operating pressures, the vaporized antimicrobial agent may becarried in a mixture with a nonreactive carrier gas such as an inert ornoble gas.

According to one aspect of the invention, water vapor is also mixed withthe gaseous antimicrobial agent to enhance its sterilizing action. Ithas been discovered that with vaporized peracetic acid as theantimicrobial agent, a relative humidity of 20-100% further enhances theeffectiveness.

Optionally, the pressure is such that the antimicrobial agent ismaintained at substantially the maximum concentration supported by thetemperature in the sterilization chamber without occurrence ofcondensation. Thus the partial pressure of the antimicrobial agent isnear the saturation vapor pressure of the antimicrobial agent for thetemperature in the sterilization chamber.

In one embodiment, the pressure of the antimicrobial agent and thecarrier gas introduced into the sterilization chamber is pulsed.Preferably the pressure pulses are in the range of from 0.1 to 50 torr.This helps to replenish antimicrobial agent consumed by reaction withthe article. It also helps to drive the antimicrobial agent intopackaging barrier or to diffuse into long lumens of medical devices,such as flexible endoscopes or hypodermic needles.

In another embodiment, the pressure is ramped up (i.e. monotonicallyincreased) during the predetermined exposure period. In general pulsinghelps to drive the antimicrobial agent into the article and increasingthe pressure with time helps to replenish spent agent. Other pressureprofiles having a combination of pulsing and ramping are also possible.

The term “peracid” as used herein, is defined to include well knownperacid antimicrobial agents such as saturated and unsaturatedperalkanoic acids including peraralkanoic acids having from 1 to 8carbon atoms and halogenated derivatives thereof. Examples of suitableperacids include known peracetic acid, halogenated peracetic acids,performic acid, perpropionic acid, halogenated perpropionic acids,perbutanoic acid and its halogen derivatives, perisovaleric acid and itshalogen derivatives, percapronic acid and its halogen derivatives,percrotonic acid, monopersuccinic acid, monoperglutaric acid, andperbenzoic acid, for example. The halogenated peracids contain one ormore chloro, bromo, iodo or fluoro groups. The preferred peracids aresufficiently volatile to form an effective sporicidal vaporconcentration at temperatures less than 80° C.

It is to be understood that the operating temperature of the presentprocess is determined by the characteristics of the articles beingsterilized, not by temperature limitations of the sterilization process.Many medical articles to be sterilized will not withstand temperaturesover 60° C., while other articles such as metallic surgical instrumentsare more efficiently sterilized at higher temperatures.

In the pulsed treatment using hydrogen peroxide, the article iscontacted with hydrogen peroxide vapors produced by completelyevaporating 1 to 10 (wt/wt) % hydrogen peroxide solution and preferablyfrom 2 to 8 (wt/wt) % hydrogen peroxide solution. The optimal vaporpretreatment involves contacting the article to be sterilized withhydrogen peroxide vapor in the sterilizing chamber. A total pulsedcontact time of from 5 to 30 minutes is usually sufficient to insurecontact of the entire surface of a packaged article with the hydrogenperoxide vapor.

In the pulsed treatment with peracid, peracid treatment is effected bycontact of the article with antimicrobial concentrations of the peracidvapor. Preferably, the pulsed peracid pretreatment is carried out byexposing the article to be sterilized to peracid vapor produced bycompletely evaporating from 1 to 35 (wt/wt) % peracid solution andpreferably from 6 to 12 (wt/wt) % peracid solution for a time sufficientto permit contact of the vapor with all surfaces of the article beingsterilized, packaged or unpackaged. The total pulsed contact exposuretime is preferably from 5 to 30 minutes with packaged articles. Fortemperature-sensitive articles, the peracid exposure can be carried outat a temperature of from 20 to 80° C. and preferably from 40 to 60° C.Treatment at higher temperatures is possible for articles that cantolerate them.

Some peracids in certain concentrations are explosive at elevatedtemperatures. For this reason, peracetic acid is usually transported andstored in aqueous solutions having less than 35 wt. % peracetic acid.The peracetic acid solution is easily vaporized, and effective treatmentof articles at room temperature, according to this invention, can beachieved by exposing the articles to peracetic acid vapors at partialpressures in the range of from 0.2 to 18 torr. The lower pressure limitis the lower range limit of the effective concentration of the peraceticacid, and the upper limit is the saturation vapor pressure at roomtemperature. If the article to be sterilized can tolerate highertemperatures, then the pressure range will change accordingly.

After the pulsed treatment, the mixture gas carrying the antimicrobialagent is removed by evacuating the sterilization chamber. Plasma gas isthen introduced into the sterilization chamber to sterilize the articleinside.

The particular pulse-vacuum sequence order can be reversed as desiredduring successive repetitions of the pulsed treatment and plasmatreatment cycle.

In the preferred process of this invention, the pulsed antimicrobialagent pretreatment is carried out with vapor introduced into thesterilizing chamber, and the article is pretreated with the peracidprior to exposing the article to the plasma. Suitable plasma sterilizingsystems for carrying out the process of this invention are described inU.S. Pat. Nos. 5,115,166 and 5,178,829. The entire contents of these areincorporated herein by reference.

The process of this invention uses a plasma made from gas mixturescontaining argon, helium and/or nitrogen; and oxygen and/or hydrogen,optionally containing inert gases and carbon dioxide. Nitrogen is notpreferred because it can form toxic residues. The exhaust gas productsfully satisfy current environmental and worker safety concerns, theproducts of the plasma being almost entirely water vapor, carbon dioxideand non-toxic gases normally found in the atmosphere.

Generally, a plasma is generated with an initial large component of highenergy ions and ultraviolet (UV) emission as a matter of course. As theplasma is transported down stream and out of the plasma generatingenergy field, the charged particles recombine by collision withcontainer surfaces to form uncharged energized free radicals, atoms andmolecules.

An important feature of the present invention is to avoid the use ofplasma having a large component of ions and ultraviolet emissions toeffect sterilization. Instead, uncharged species of oxidizing orreducing agents, made highly reactive by activation with the plasma, areused to effect sterilization by a chemical process.

The apparatus disclosed herein is capable of producing plasma havinguncharged, highly reactive species. For example, in the plasmagenerating chamber, oxygen is energized by microwave radiation and formsa plasma having an initial high concentration of ions and ultravioletemissions. These are not allowed into the sterilization chamber as theytend to be strongly corrosive on the article to be sterilized, or thepackaging. The UV emissions are localized in the plasma generatingchamber and are attenuated by the restriction means and the plasmadistribution means before they reach the sterilizing chamber. Similarly,as high energy ions hit the restrictive means and the internal walls ofthe plasma distribution means, they recombine with free electrons torevert to highly reactive uncharged atoms and radicals. By the time theplasma enters the sterilizing chamber, the plasma's downstream productsconsist essentially of energized and highly reactive uncharged freeradicals and electronically excited uncharged atoms and molecules.

Typically, a microwave source is used to generate the plasma. It ischanneled by a waveguide to form a highly confined EM field zone. Littleof that field can spread to the sterilizing chamber. Thus, production ofhigh energy ions and UV is only possible in the field region of theplasma generating chamber and not outside of it. Also, there is no EMfield to cause non-uniformity in the sterilizing chamber. Therestriction means of Applicants' apparatus, apart from obstructing thepassage of UV and ions as noted above, further helps to make plasmageneration outside the plasma generating chamber less favorable. Therestriction means maintains an optimal gas pressure in the plasmagenerating chamber for generating plasma. Once the gas exits via therestriction means, the pressure drops to make generation difficult undernormal conditions even if the EM field of the microwave source extendedinto this region. Thus, UV and ions can only be generated in the plasmagenerating chamber; once outside, they are allowed to dissipate to forma downstream plasma consisting essentially of energized, highly reactiveuncharged free radicals, atoms and molecules.

Under these conditions, effective sterilization is effected withoutsignificant deterioration of packaging materials in which articles to besterilized may be placed.

One suitable apparatus is. shown in FIG. 1. FIG. 1 is a top view andFIG. 2 is a front view of a single waveguide plasma sterilizerembodiment of this invention. The plasma sterilizer has a plasmagenerator 2 and a sterilizing chamber 4. The plasma generator 2comprises an electromagnetic field generator such as a magnetron 6 and awaveguide 8 which directs the electromagnetic field. The plasma sourcegases are directed into plasma generating and delivering tubes 10, 12,and 14 by feeder tubes from gas delivery tubes 16, 18 and 20 leadingfrom the control valve complex 22. Individual gases are fed from thepressured gas sources (not shown) by inlet lines 24, 25 and 26. Theoperation of the control valves in valve complex 22 is controlled by thecentral processing unit (CPU) 28 by standard procedures. The controlvalves and CPU can be any of the conventional, standard devices used forgas flow control in plasma generating equipment.

The sterilizing chamber 4 comprises top plate 30, side plates 32 and 34,bottom plate 36, back plate 37 and front sealing door 38 through whicharticles or materials to be sterilized are placed in the chamber. Theplates are attached together in a sealed relationship to form a vacuumchamber, such as by welding. The door 38 is secured in a sealedrelationship with the sterilizing chamber. It is hinged at the top, sideor bottom with conventional hinge pins (structure not shown) to swingagainst abutting surfaces and an O-ring seal 40 (FIG. 3) of the side,top and bottom plates, where the pressure difference between theinternal chamber vacuum pressure and the surrounding atmosphericpressure holds it tightly in place.

The plates and door can be made of any material having the strengthrequired to withstand the external atmospheric pressure when the chamberis evacuated. Stainless steel or aluminum plates and door are preferred.The internal surface material of the chamber is critical and greatlyaffects the number of killing species available in the chamber. Asuitable material is pure (98%) aluminum which can be applied either asa liner or as a flame-sprayed coating on all internal walls of thestainless steel chamber. An alternate material is nickel. An optimummaterial is an inert polymer such as polytetra-fluoroethylene (TEFLON).

Antimicrobial additives are added as a liquid or vapor through conduit35 to inlet port 39 (FIG. 4).

The gases are exhausted from the sterilizing chamber through exhaustoutlet port 42 to a conventional vacuum pump system (not shown).

FIG. 3 is a top cross-sectional view of the plasma sterilizer embodimentof FIG. 1 and FIG. 2, taken along the line 3—3 in FIG. 2. FIG. 4 is aside cross-sectional view of the plasma sterilizer embodiment of FIG. 1and FIG. 3, taken along the line 4—4 in FIG. 3. Each of the plasmagenerators 10, 12 and 14 comprise an inlet cap 44 with gas inlet ports46, 48 and 50 leading to a respective gas generator tube 51, 52 or 53leading through the waveguide 8. In the waveguide 8, the gases areenergized and convert in tubes 51, 52 and 53 to a plasma. The gasgenerator tube directs the plasma flow into the gas distribution tubes54, 56 and 58 from which the plasma is fed into the sterilizing chamber60. The gas generator tubes are enclosed in tubular metal cooling tubes62 and 64. The caps 44 and the cooling tubes 62 and 64 are preferablyprovided with grooves or cooling fins (not shown) in a conventionalmanner to increase their efficiency in removing heat from gas generatortubes.

The distal end of tubes 51, 52 and 53 have increased thickness and formsa smooth surfaced venturi restriction 96 of reduced cross-sectionalarea. Cap 98 positioned on the proximal end of plasma distribution tube56 has a preselected restrictive opening 99 of further reducedcross-sectional area. These restrictions are critical aspects of thepreferred embodiment of this invention, creating a pressure differencebetween the low pressure plasma generating zone 87 and the vacuumpressure in the distribution tube 56 and sterilizing chamber 60, therebyallowing the operating pressures in the two chambers to be independentlyoptimized.

The diameter of the restrictive opening 99 is selected to maintain aback pressure of from 0.1 to 150 torr and preferably from 1 to 40 torrin the plasma generating zone, with a vacuum chamber pressure in therange of from 0.01 to 100 torr and preferably from 0.1 to 15 torr. Thispressure provides optimum energy transfer from the electromagnetic fieldto the gases with gas mixtures containing oxygen; argon, helium and/ornitrogen; and/or hydrogen and is a major factor for the production of ahigh yield of plasma at a minimum temperature and with the minimum powerrequirement achieved with the device of this invention. For mostoperating parameters, the restriction 99 can have a diameter of from4.82 to 8.00 mm and preferably from 6.28 to 6.54 mm. These dimensionsmay also vary if gas flows, vacuum system or the conductance of the gasdistribution system changes.

The distal ends of the gas distribution tubes 54, 56 and 58 aresupported by spring-biased end supports 66 mounted on sideplate 32.

The door 38 is held in sealing engagement by atmospheric pressureagainst the O-ring seal 40 mounted in the flange 41 extending from theside plates 32 and 34, and the top and bottom plates 30 and 36 (notshown). Optionally, additional conventional closure clamps or latchescan be used to insure closure of the door before chamber evacuation isinitiated.

FIG. 5, FIG. 6 and FIG. 7 are cross-sectional views of gas distributiontubes 54, 58 and 56, respectively, showing angular positions of the gasdistribution outlet ports. The outlet ports are positioned to provideplasma flow to all lower portions of the sterilizing chamber 60 wherearticles to be sterilized are placed. Tube 54 shown in FIG. 5 is placedadjacent back plate 37 and directs plasma gases downward and toward thelower center of the chamber through outlet ports 70 and 72,respectively. Tube 58 shown in FIG. 6 is placed adjacent the door 38 anddirects plasma gases downward and toward the lower center of the chamberthrough outlet ports 74 and 76, respectively. Tube 56 shown in FIG. 7 isplaced in the central portion of the chamber 60 and directs plasma gaseslaterally downward through outlet ports 78 and 80. The outlet portsshown for the distribution tubes are representative and can be changedto any other configuration which achieves optimal plasma distribution tothe sterilizing zone or zones of the chamber. Although only one angulararrangement is shown, each tube can have more than one angular set ofoutlet ports, each having different angles, along the length of thetube, as desired. The choice of outlet port angles and locations shouldbe selected in view of how the articles to be sterilized are to beplaced in the chamber and the type of article to be sterilized.

The plasma is directed through a change of direction, preferably atleast 90°, before discharging it into the sterilizing chamber. Thisprevents direct impingement of hot plasma onto the articles beingsterilized. The gas distributors also allow ions to recombine bycollisions with their surfaces and block the UV radiation.

The apparatus can be used to generate a sterilizing plasma from amixture of oxygen; argon, helium, and/or nitrogen; and hydrogen, or witha mixture of air and hydrogen, supplemented by oxygen or nitrogen togive the desired ratios. The sterilization is carried out at a vacuumpressure of from 0.01 to 15 torr and preferably from 1 to 15 torr.Sterilization may be carried out at higher pressures provided steps aretaken to ensure uniformity of gas flows and temperature throughout thechamber. The temperature in the sterilizing chamber is maintained below80° C. and preferably from 38 to 60° C. for articles that can nottolerate high temperatures. Elevated temperatures may preferably be usedwith articles capable of withstanding them.

Typically, a microwave source is used to generate the plasma. It ischanneled by a waveguide to form a highly confined electromagnetic (EM)field zone. Little of that field can spread to the sterilizing chamber.Thus, production of high energy ions and UV is only possible in thefield region of the plasma generating chamber and not outside of it.Also, there is no EM field to cause non-uniformity in the sterilizingchamber. The restriction means, apart from obstructing the passage of UVand ions as noted above, further helps to make plasma generation outsidethe plasma generating chamber less favorable. The restriction meansmaintains an optimal gas pressure in the plasma generating chamber forgenerating plasma. Once the gas exits via the restriction means, thepressure and the EM field drop to make generation difficult under normalconditions. Thus, UV and ions can only be generated in the plasmagenerating chamber; once outside, they are allowed to dissipate to forma downstream plasma consisting essentially of energized, highly reactiveuncharged free radicals, atoms and excited molecules.

Under these conditions, effective sterilization is effected withoutsignificant deterioration of packaging materials in which articles to besterilized may be placed.

Following pulsed treatment with antimicrobial agent, the sterilizingchamber is evacuated to a pressure of less than 10 torr. The article isthen exposed to a plasma generated from a gaseous mixture of argon,helium or nitrogen mixed with oxygen and/or hydrogen at temperatures ofless than 60° C., a pressure of from 0.01 to 100 torr, preferably from0.1 to 15 torr and a treatment time of at least 5, and preferably from10 to 15 minutes. For sterilizing packaged goods, the gas mixtures fromwhich the plasma is generated can contain from 1 to 21 (v/v) % oxygenand from 1 to 20 (v/v) % hydrogen, the balance being argon, heliumand/or nitrogen and optional small quantities of inert gases.

It is to be understood that the operating temperature of the presentprocess is determined by the characteristics of the articles beingsterilized, not by temperature limitations of the sterilization process.Many medical devices will not withstand temperature over 60° C., whileother articles such as metallic surgical instruments are moreefficiently sterilized at higher temperatures.

Similarly, the pressure limitations given are examples illustrative ofthe preferred embodiments. Different pressure limits are contemplatedfor other plasma generating chambers or sterilizing chambers havingdifferent dimensions or surface characteristics.

The gas mixtures producing plasmas for sterilizing packages preferablycontain from 1 to 10 (v/v) % oxygen and from 2 to 8 (v/v) % hydrogen,and optimally contain from 2 to 8 (v/v) % oxygen and from 3 to 7 (v/v) %hydrogen. Packages are treated for at least 15 minutes and preferablyfrom 1 to 5 hours.

In an optimum method of sterilizing, the articles to be sterilized areplaced in the sterilizing chamber, supported by conventional fixtureswhich permit the plasma gas products to reach all surfaces of thearticles.

1. The chamber is closed. The sterilizing chamber is evacuated to apressure of 0.1 torr, and an isolation valve between the vacuum pump andsterilizing chamber is closed.

2. Antimicrobial agent in the form of a pulse of vaporized peraceticacid in a gaseous mixture with argon and/or water vapor as a carrier gasis admitted into the chamber. The peracetic acid vapor has aconcentration of at least 0.1 mg/L peracetic acid. Alternatively, apulse of vaporized hydrogen peroxide in a gaseous mixture with argon asa carrier gas is introduced into the chamber. The hydrogen peroxidevapor has a concentration of at least 0.1 mg/L hydrogen peroxide. Ifperacetic acid pretreatment is to be combined with hydrogen peroxidepretreatment, vaporized peracetic acid and hydrogen peroxide in agaseous mixture with argon and/or water vapor as a carrier gas isadmitted into the chamber. The peracetic acid vapor has a concentrationof 0.1 mg/L peracetic acid. The hydrogen peroxide vapor has aconcentration of at least 0.1 mg/L hydrogen peroxide. The pulse ofvaporized peracetic acid and/or hydrogen peroxide vapor exposure iscontinued for from 5 to 30 minutes.

3. The pressure in the sterilizing chamber is reduced to less than 2torr for from 1 to 2 minutes.

4. Steps 2 and 3 are repeated 4 times, and the chamber is evacuateduntil a predetermined amount, preferably at least 90% of theantimicrobial agent has been removed.

5. Process gases are admitted to the plasma chamber, preferably at aflow rate of up to 5 liters per minute, and optimally from 3 to 4 litersper minute.

6. The magnetron is energized to create the plasma, and the plasmaproducts flow into the sterilizing chamber.

7. The plasma treatment is continued for from 5 to 30 minutes andpreferably from 5 to 15 minutes.

8. The magnetron is deactivated and the process gas flow to the plasmachamber is terminated.

9. Steps 1-8 are repeated until sterilization is complete and all sporesare killed. Hydrogen peroxide and peracetic acid treatments can bealternated so that the pretreatment is limited to either one for aparticular cycle repetition.

10. The isolation valve between the pump and chamber is closed, and thechamber is vented to the atmosphere. The sterilizing chamber can bepumped down and partially vented to remove acidic vapors before beingfully vented to the atmosphere.

The above method sterilizes effectively in less time than is requiredwithout the pulsed antimicrobial agent treatment.

The remaining reactive plasma components have a short life, and quicklydecay to form water vapor (gas), carbon dioxide, and other non-toxiccomponents usually found in air. These are fully acceptable as residuesor as exhaust gas components.

One application of the present invention is to sterilize lyophilizers.Lyophilizers are essentially large vacuum chambers (e.g., 5 to 30 cubicft.) used to freeze dry solutions or liquid suspensions. In the medicalindustry they are used primarily to freeze dry pharmaceuticals forpreservation.

FIG. 8 is a schematic illustration of a lyophilizer adapted to besterilized by the plasma process of the present invention. Thelyophilizer system 300 includes a lyophilizer chamber 312, a condenser314 and a vacuum pump 316. The vacuum pump 316 is capable of evacuatingthe lyophilizer chamber 312 via a vacuum line 322, the condenser 314 andanother vacuum line 324. Typically, the material or articles to be dried(which may be frozen) are held in containers and loaded into thelyophilizer chamber 312. After the door or the lyophilizer chamber (notshown) is sealed, the interior of the lyophilizer is evacuated using thevacuum pump 316. The material or articles cool (and freeze, if they arenot already frozen) as water evaporates. The evaporated water is mostlycondensed by the condenser 314 and removed. Further pumping causes waterin the material or articles to sublime until substantially all the wateris removed. When the process is complete, the lyophilizer chamber 312 isvented and the containers are removed.

Lyophilizers used to prepare pharmaceuticals must have their interiorchamber sterilized after each batch operation. Conventional methods ofusing heat or ethylene oxide are problematic. As mentioned earlier,ethylene oxide is a carcinogen and great precautions must be exercisedin its use. Adapting the lyophilizer to steam sterilization raisesconsiderations such as condensation problems and the ability of thelyophilizer to withstand high temperature (e.g., 121° C.) and highinternal pressure (e.g., 15 psig).

According to one aspect of the present invention, a plasma generatingsystem 330 includes a plasma generating chamber 332. The lyophilizerchamber is connected to receive a gas plasma from the plasma generatingchamber 332 via a port 340 and connecting tube 342. During the plasmasterilization process, the lyophilizer chamber acts as a sterilizingchamber with the evacuation provided by the vacuum pump 316.

In one embodiment, the interior of the lyophilizer system is sterilizedby the process of the invention. In another embodiment, both theinterior of the lyophilizer system and the material or articles thereinafter completion of lyophilization are subjected to the process of theinvention for sterilization.

For a large capacity lyophilizer, distributing gas plasma via a singleport or location to the interior of the lyophilizer chamber 312 may ledto nonuniformity. One or more internally fitted gas distributors such asthat shown in FIGS. 3, 4 are preferable. In other embodiments,additional ports such as 340′, 340″ and 340′″ and additional connectingtubes such as 342′, 342″ and 342′″ connected to the plasma generatingchamber 332 are used to introduce the gas plasma into various regions ofthe lyophilizer chamber 312.

The antimicrobial agent discussed previously is introduced from anantimicrobial agent source 350 through a line 352 into the lyophilizerchamber 312 via one or more of the ports and connecting tubes. In thisway, the process of the aforementioned invention can be practiced.

FIG. 9 is a schematic illustration of a general enclosure adapted to besterilized by the plasma process of the present invention. The enclosure412 is a rigid structure able to withstand evacuation. It has one ormore inlet ports, such as 440, 440′, 440′″, and one or more outlet portssuch as 426, 426′. A vacuum pump 416 is capable of evacuating theenclosure 412 through one or more of the outlet ports such as 426, 426′via vacuum lines such as 424, 424′.

The antimicrobial agent is introduced from an antimicrobial agent source450 through a line 452 into the enclosure 412 via one or more of theinlet ports and connecting tubes such as 442, 442′, 442″, 442′″.

A plasma generating system 430 includes a plasma generating chamber 432in which plasma is generated from a gas mixture by coupling of energyinto it. A suitable plasma generating system has previously beendisclosed. Another suitable plasma generating system has also beendisclosed in U.S. Pat. No. 5,115,166, the entire content thereof isincorporated herein by reference. The enclosure 412 is connected toreceive a gas plasma from the plasma generating chamber 432 via one ormore of the inlet ports and connecting tubes such as 442, 442′, 442″,442′″.

In this way, the enclosure illustrated in FIG. 9 is capable of beingsterilized by exposure to antimicrobial agent as well as to a plasmagas.

While the embodiments of the various aspects of the present inventionthat have been described are the preferred implementation, those skilledin the art will understand that variations thereof may also be possible.Therefore, the invention is entitled to protection within the scope ofthe appended claims.

What is claimed is:
 1. A process for plasma sterilization of an interiorof a chamber and any articles therein, comprising: exposing saidinterior of said chamber and any articles therein to at least onecombination sterilizing cycle, each combination sterilizing cyclecomprising: a pulsed treatment with gaseous antimicrobial agent, saidpulsed treatment comprising one or more pulse-vacuum cycles, eachpulse-vacuum cycle comprising the steps of evacuating said chamber andexposing the interior of said chamber and any articles therein togaseous antimicrobial agent for a predetermined duration; removing theantimicrobial agent after the pulsed treatment by evacuating saidchamber; and a plasma treatment comprising exposing the interior of saidchamber and any articles therein to a stream of plasma, said plasmabeing generated in a separate plasma generating chamber and beingsupplied to effect sterilization in said chamber.
 2. The process ofclaim 1, wherein the antimicrobial agent is selected from the groupconsisting of hydrogen peroxide, a peracid antimicrobial agent, ormixtures thereof, the peracid antimicrobial agent being selected fromthe group consisting of saturated and unsaturated peralkanoic acidshaving from 1 to 8 carbon atoms and halogenated derivatives thereof. 3.A. The process of claim 2 wherein the peracid antimicrobial agent isperacetic acid.
 4. The process of claim 2 wherein the antimicrobialagent is halogenated peracetic acid.
 5. The process of claim 2 whereinthe antimicrobial agent is hydrogen peroxide.
 6. The process of claim 2wherein the antimicrobial agent is a mixture of peracetic acid andhydrogen peroxide.
 7. The process of claim 1, wherein the pulsedtreatment and the plasma treatment in each combination sterilizing cyclefollow a predetermined order.
 8. The process of claim 1, wherein thepulsed treatment is preceded by a plasma treatment.
 9. The process ofclaim 1, wherein the step of exposing the article to gaseousantimicrobial agent for a predetermined duration includes ramping thepressure of the gaseous antimicrobial agent.
 10. The process of claim 1,wherein the gaseous antimicrobial agent is carried in a gaseous mixturewith a nonreactive carrier gas.
 11. The process of claim 10, wherein thegaseous antimicrobial agent in the gaseous mixture has a partialpressure of from 0.1 to 50 torr, and the gaseous mixture with theantimicrobial agent is removed after the pulsed treatment by evacuatingsaid chamber until a predetermined amount of the antimicrobial agent hasbeen removed.
 12. The process of claim 10, wherein the gaseousantimicrobial agent in the gaseous mixture in said chamber is maintainedat a predetermined temperature and at partial pressures up to a maximumconcentration of antimicrobial agent supported by said temperaturemaintained in said chamber.
 13. The process of claim 10 wherein theantimicrobial agent is selected from the group consisting of hydrogenperoxide, a peracid antimicrobial agent, or mixtures thereof, theperacid antimicrobial agent being selected from the group consisting ofsaturated and unsaturated peralkanoic acids having from 1 to 8 carbonatoms and halogenated derivatives thereof.
 14. The process of claim 10wherein the peracid antimicrobial agent is peracetic acid.
 15. Theprocess of claim 10 wherein the antimicrobial agent is halogenatedperacetic acid.
 16. The process of claim 10 wherein the antimicrobialagent is hydrogen peroxide.
 17. The process of claim 10 wherein theantimicrobial agent is a mixture of peracetic acid and hydrogenperoxide.
 18. The process of claim 10 wherein the gaseous mixtureadditionally includes water vapor.
 19. A process for plasmasterilization of an interior of a chamber and any articles therein,including: exposing said interior of said chamber and any articlestherein to at least one combination sterilizing cycle, each combinationsterilizing cycle comprising: an antimicrobial pretreatment by exposingsaid interior of said chamber and any articles therein to gaseousantimicrobial agent; removing the antimicrobial agent from said chamber;and exposing said interior of said chamber and any articles therein to astream of plasma, said plasma being generated in a separate plasmagenerating chamber.
 20. The process of claim 19, wherein the gaseousantimicrobial agent is carried in a gaseous mixture with a nonreactivecarrier gas.
 21. The process of claim 20, wherein the gaseousantimicrobial agent in the gaseous mixture has a partial pressure offrom 0.1 to 50 torr, and the gaseous mixture with the antimicrobialagent is removed after the pulsed treatment by evacuating said chamberuntil a predetermined amount of the antimicrobial agent has beenremoved.
 22. An enclosure adapted for sterilization therein, comprising:one or more inlet ports; one or more outlet ports; a vacuum pumpconnectable to at least one of said outlet ports of the enclosure forevacuating the enclosure; an antimicrobial agent source connectable toat least one of said inlet ports of the enclosure for introducing anantimicrobial agent therein; and a plasma generating system connectableto at least one of said inlet ports of the enclosure for introducing aplasma therein.
 23. An enclosure adapted for sterilization therein as inclaim 22, wherein the enclosure is a sterile isolation enclosure.
 24. Anenclosure adapted for sterilization therein as in claim 22, wherein theenclosure is a lyophilizer.
 25. An enclosure adapted for sterilizationtherein as in claim 22, wherein the enclosure contains at least onearticle.
 26. A sterilization system, comprising: an enclosure forsterilizing therein, said enclosure having one or more inlet ports andone or more outlet ports; a vacuum pump connectable to at least one ofsaid outlet ports of the enclosure for evacuating the enclosure; anantimicrobial agent source connectable to at least one of said inletports of the enclosure for introducing an antimicrobial agent therein;and a plasma generating system connectable to at least one of said inletports of the enclosure for introducing a plasma therein.
 27. Asterilization system as in claim 26, wherein the enclosure is a sterileisolation enclosure.
 28. A sterilization system as in claim 26, whereinthe enclosure is a lyophilizer.
 29. A sterilization system as in claim26, wherein the enclosure contains at least one article.